In a typical lost foam casting process, an expendable pattern formed of expanded polystyrene is embedded in an unbonded sand mold. Molten metal poured into the mold decomposes and replaces the pattern to form, upon cooling, a product casting. Decomposition of the pattern during casting generates vapors that vent into the surrounding sand mold. It is desired to make the pattern of very low density polymer foam to minimize the mass to be decomposed and the volume of the resulting vapors.
Polystyrene is commercially available in the form of a dense bead containing a blowing agent. To make the pattern, the dense bead is first pre-expanded by heating to activate the blowing agent, whereupon the bead expands to nearly final density. The pre-expanded beads are entrained in an air stream and injected and packed into a mold. Within the mold, the packed beads are subjected to steam and fused into a section that may be suitably assembled with other sections to complete a pattern.
However, problems during the injection of the polystyrene beads into the mold may cause inadequate filling of the mold cavity and produce unacceptable patterns. Clumps of the beads that form may clog the fill tube and fill gun leading to the mold cavity. Also, instead of uniformly packing to fill the mold, there is a tendency for the beads to pack in a manner that leaves voids amongst the beads, which in turn produces voids in the pattern. I have now discovered that these problems are related to the accumulation of static electrical charge on the beads during pre-expansion and storage. Also during filling of the mold cavity, the static charge produces attractive and repulsive forces between the beads which are enhanced because of the low density of the beads.
It is an object of the present invention to provide an apparatus and method for neutralizing static electrical charge carried by low-density pre-expanded polystyrene beads during entrainment in an air stream for injection into a mold and steam fusion into an integral structure such as a lost foam pattern, which method comprises treating the air stream prior to contact with the beads to generate ions therein effective to counteract the static charge and thereby promote complete and uniform filling of the mold to produce a void-free structure.
Yet another object of the present invention is to provide an apparatus and method for making polystyrene patterns which: (1) require less time to make the polystyrene pattern, (2) have uniform fusion times and pressures and (3) allow the use of lower pressures.